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Squiggly-line-land view of the Earth. What’s going on in the upper mantle ? Receiver function , powerful seismic tool What in the world does the structure of the inner core mean? Is it still rotating , like it was in 1996?. Outline of mantle discussion. (I just got a digital camera).

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squiggly line land view of the earth
Squiggly-line-land view of the Earth
  • What’s going on in the upper mantle?
    • Receiver function, powerful seismic tool
  • What in the world does the structure of the inner core mean?
  • Is it still rotating, like it was in 1996?
outline of mantle discussion
Outline of mantle discussion

(I just got a digital camera)

  • USArray
  • Receiver function analysis
  • MOMA
  • Africa
  • RISTRA
  • The upper mantle discontinuities
    • Water at 410-km-depth
    • A double “520”
earthscope components
EarthScope Components
  • EarthScope's facilities include the following four coupled components:
    • USArray (United States Seismic Array)
    • SAFOD (San Andreas Fault Observatory at Depth)
    • PBO (Plate Boundary Observatory)
    • InSAR (Interferometric Synthetic Aperture Radar)
why look at the upper mantle
Why look at the upper mantle?
  • Mapping seismic structure
    • P & S velocity, density, anisotropy
  • To deduce physical characteristics
    • Chemical and thermal heterogeneity
  • To deduce what’s going on
    • Stagnant or moving continental keels
    • Dynamics of upper thermal boundary layer of the mantle
    • Mantle circulation
seismic style study
Seismic-style study
  • Reflection for crustal structure
  • S-wave splitting for anisotropy
    • Flow direction - aesthenosphere
    • Relic fabric - lithosphere
  • Surface and body wave tomography
    • Absolute velocities in upper few 100 km
  • Body wave tomography (deeper)
  • Receiver functions
    • Best resolution of radial velocity gradients
the receiver function
The receiver function
  • Pioneered by seismologists including Bob Phinney and Chuck Langston
  • Examines echoes of the P wave to determine zones of high radial gradient in seismic velocity
  • It is proving to be a very useful companion to seismic tomography, providing detailed pictures of near-receiver structure
slide13

Ray paths contributing

to receiver functions

Chuck Ammon

slide14

Radial component

of receiver function

Just useful for finding the Moho

mechanics of a receiver function
Mechanics of a receiver function
  • Extract the P wave from the vertical component
  • Deconvolve it from the horizontal component
  • This should leave a spike at the P arrival time and a string of P-S conversions
  • Convert the conversions (as a fcn of time and ground motion) to structure (impedance as a function of depth)
  • Average together the records from many distances and azimuths
some limitations
Some limitations
  • Assumes no lateral variations in structure
    • Migration can overcome this limitation
  • Only works in a frequency pass band
    • Cannot recover baseline, trends, or really much beyond about 100-200 km wavelength velocity structure
    • Generally falls apart shorter than 5-10 km wavelengths
slide18
MOMA
  • Missouri to Massachusetts transect
  • 19 stations placed every 100 km
  • Chosen for nice graphics

Mike Wysession

Keith Koper

moma discontinuity imaging
MOMAdiscontinuity imaging

Mike

Wysession

Karen

Fischer

slide21

Stereo vision

Receiver functions

from events to the

north

East!?

West

Events

to the south

slide22

Tomography plus receiver functions

T < 150° C

Disagreement with individual profiles

Farallon depression?

slide25

Again, well-resolved reflections from near 410 and 660

Note the presence of clear 410 conversions at short-period

thicker transition zone to ne
Thicker transition zone to NE

Transition thickness near global average of 245 km, so not cold under region, 10 km of relief may correspond to ~60° temperature difference

cooler

warmer

receiver function migration
Receiver function migration
  • Just like migrating seismic reflection data
  • Benefits from adequate spatial sampling
  • Ability to image structure depends on
    • Depth of structure
    • Frequency of waves recorded
  • Of course, more events with more back-azimuths, and more distances are helpful
slide28

Resolution with

70 km spacing

T= 15s

slide29

Resolution with

10 km spacing

T= 2s

slide33

MOMA migration

MOMA Array: Depth Migration LP10s

cheyenne belt receiver functions

XD

SS

CB

GF

Moho

Moho

SLAB

Cheyenne Belt Receiver Functions

Imbricated Moho

Mantle layered

Archean

Mantle

Modified

Proterozoic

Mantle

Fast from tomography

From Ken Dueker

slide35

RISTRA

Rio Grande Rift

Ran from Texas into Utah

Rick Aster

Receiver functions across the 1000-km line give a good picture of the shallow structure, and show little topography on the 410 and 660.

moho

hot off the jgr press

Ken Dueker

Hot off the JGR “press”
  • Hersh, Dueker, Sheehan, and Molnar, JGR
  • 410 and 660 topography under western US
  • 20-30 km topography, with 500 km scale length
  • No relation to surface tectonics
  • Sharpness not easily related to depth
  • Conclusions:
    • Either transition zone has smaller scale convection than deep mantle
    • Or there is a lot of compositional variation down there
slide38

Anne Sheehan

Field area

slide40

410 topography

+/- 10 km

660 topography

+/- 15 km

slide42

Science 6 June 2003

Seismic evidence for water

deep in the Earth’s upper mantle

Federica Marone

Mark van der Meijde

Domenico

Suzanne van der Lee

slide43

Science 6 June 2003 - van der Meijde et al.

1000 ppm water broadening the 410-km-discontinuity?

main points of van der meijde
Main points of van der Meijde
  • Conversion from “410” stronger at low frequency than high, but conversion from “660” is steady
  • So “410” must be broader, in fact very broad, 20-40 km wide
  • Subduction has been pervasive, so water might be common near 410-km-depth
  • Entire story is consistent if about 1000 ppm water is present.
slide45

~1 s period

6 s period

slide46

9 stations

The general trend is consistent, and statistics can be constructed to support the significance of the trend.

slide48

The phase P’P’

Jim Whitomb

DLA

jgr fei vidale and earle
JGR, Fei, Vidale and Earle
  • Rounded 3 good datasets of P’P’
    • California networks
    • LASA recordings
    • Highly selected GSN seismograms
  • We’ll see
    • Sharp 660-km-depth discontinuity
    • Somewhat less sharp 410, sometimes
    • (but MUCH sharper 410 than claimed for Europe)
    • No 520
slide53

More 660

than 410 energy,

Nothing else

Fei Xu

slide54

Comparison to long-period reflections

Corrected for attenuation

this means
This means
  • 660 sharp enough to efficiently reflect 1 Hz waves - less than 2 km thick transition
  • 410 not so sharp - our data is fit by half a sharp jump, half spread over 7 km
slide58

Science, 2001. Sees 520 sometime simple, sometimes split.

Arwen

Deuss

Interprets this as the 520 having phase changes in two components, olivine and garnet, whose depths don’t always coincide.

(Also has claims to see PKJKP

and a “250”)

slide60

Transects

of the 520

Lateral continuity of structure

some high points
Some high points
  • “410”
    • Why is it’s brightness variable?
    • Can we map the pattern globally to learn more?
    • Is topography real?
  • “520”
    • Why does it flicker?
  • “660”
    • Is topography a thermometer?
  • Other discontinuities?
  • Better images on the way from USArray
the enigmatic inner core
The enigmatic inner core
  • Layering
  • Anisotropy
  • Rotation
  • Possible origins of structure
  • Combined my slides with those of Ken Creager and Shun Karato

Some slides lent by

Ken Creager and Shun Karato

seismic characteristics of the inner core
Seismic characteristics of the inner core
  • A large Poisson’s ratio, close to that of a liquid
  • High attenuation (Qs~100-200)
  • Strong anisotropy
slide65

A current working model

Upper Inner Core:

Isotropic, finely heterogeneous

West: 0.8% slower

250 km thick

Q = 600

East: thicker

Q = 250 in east

Middle Inner Core:

Strong anisotropy

Isotropic Voigt average is homogeneous

Innermost Core:

Different anisotropy?

Isotropic Upper Inner Core

Transition Region

IMIC

Anisotropic Lower

inner Core

slide67

Comparing polar and equatorial data

Ouzounis and Creager, GRL, 2001

slide68

Beghein and Trampert

Science, 2003

Adam and

Miaki Ishii

summed slant stack
Summed slant stack

(Vidale & Earle)

slide70

Proposed mechanisms of inner core anisotropy

Convective flow due to high Rayleigh numberaligns crystals (most effective near surface)

Jeanloz & Wenk, GRL, 1988

slide71

Inhomogeneous growth of inner coredrives convective flow that restores isostatic equilibrium

Yoshida et al., JGR, 1996

slide72

Dendritic growth of crystals aligns a-axes radially with heat flow direction (assumes c-axis is fast)

Michael Bergman, Science, 1997 (modified by Michael Wysession)

slide73

Strong heterogeneities, various crystal alignment orientations

Modified from Annie Souriau, Science, 1998

slide74

Rotationally wrapped magnetic field around inner corecauses Maxwell stresses that align crystals (c-axes cylindrically radially out)

Bruce Buffett, Nature, 2001

slide75

Lorentz forcesproduce axisymmetric, sustained flow that aligns crystals

Modified from Shun-Ichiro Karato, Nature, 1999

hemispherical asymmetry
Hemisphericalasymmetry

Sumita and Olson (1999)

Hemispherical asymmetry might be due to heterogeneous thermal boundary conditions at the inner-core boundary caused by core-mantle interaction.

[Time-scale for anisotropic structure formation must be comparable to or shorter than the time scale for changes in mantle structure.]

does the inner core rotate with respect to the mantle
Does the inner core rotate with respect to the mantle?

Song and Richards, 1996 yes 1.1 deg/yr

Creager, 1997, yes 0.2-0.3 deg/yr

Vidale et al., 2000, yes 0.15 deg/yr

Song, 2002, yes 0.5-1.0 deg/yr

Laske and Masters , 2002, maybe 0.13±0.11 deg/yr

Souriau, 2001, no, at least not very fast, <0.1 - 0.2 deg/yr

why do we care
Why do we care?
  • I think it’s interesting
  • Would mean the core has either
    • Quite low viscosity
      • Can deform fast enough to keep moving
    • Quite low viscosity
      • Deforms so little that there is little viscous drag
  • Would prevent association of IC structure with mantle structure
25 years of data
25 years of data

Xiao-Dong Song and Paul Richards

song agu monograph 2002
Song, AGU Monograph, 2002

More Sandwich doublets

bottom line inner core may lap earth every 2000 years
Bottom line:Inner core maylap Earth every2000 years

Wild card - Does the outer core

change over time?

quick review
Quick Review
  • Mantle discontinuities still remain interesting after 40 years
  • Inner core is being mapped but not yet understood
  • Inner core is likely turning slowly
  • Seismology and mineral physics must progress together